An automatic transmission mounted on a vehicle has a speed change mechanism that is coupled to an engine via a torque converter or the like and has multiple power transmission paths. A belt-type Continuously Variable Transmission (CVT) is an example of such a speed change mechanism. The belt-type continuously variable transmission has a drive pulley (input shaft pulley or primary pulley) and a driven pulley (output shaft pulley or secondary pulley) each having a V-shaped pulley groove, as well as a belt wound around them. When a groove width of the pulley groove of one of the pulleys increases and simultaneously the groove width of the pulley groove of the other pulley decreases, effective radii of the belt wound around the respective pulleys continuously change so that the speed ratio can be continuously set. Each pulley is formed of a fixed sheave and a movable sheave. A hydraulic actuator arranged on a rear side of the movable sheave can axially move the movable sheave to change the speed ratio.
In the belt-type continuously variable transmission thus configured, it become difficult to perform the change operation of the speed ratio when a failure occurs in a solenoid controlling a hydraulic pressure supplied to the hydraulic actuator. In view of this, for example, Japanese Patent Laying-Open No. 8-326855 has disclosed a failure diagnosis device for an automatic transmission that can precisely perform failure diagnosis of the automatic transmission. This failure diagnosis device includes control signal constant-state sensing means for sensing a state in which a control signal is substantially constant, hydraulic pressure sensing means for sensing a controlled hydraulic pressure, and first failure diagnosis means for determining that the automatic transmission or the hydraulic pressure sensing means has a failure when the control signal constant-state sensing means senses the control signal in the substantially constant state and the hydraulic pressure sensed by the hydraulic pressure sensing means changes to an extent equal to or larger than a predetermined extent.
The failure diagnosis device disclosed in the above publication can perform the failure diagnosis even in the state where change in speed ratio, belt slip or the like does not occur. This can increase opportunities of the diagnosis as compared with a conventional failure diagnosis device so that the failure diagnosis can be performed early, and the possibility of damage of the automatic transmission can be reduced.
However, the belt-type continuously variable transmission has a plurality of solenoids. Therefore, it is necessary to specify a location of the failure for precisely determining the failure. The failure diagnosis device disclosed in the foregoing publication merely determines the presence/absence of the failure, and no consideration is given to specific determination of the location of failure.